The apparatus and method of this invention relate to the field of making capsules having a core material encased within a shell or wall material. Encapsulation is the term applied to the formation from suitable materials of a shell which encloses a core material. The capsule that is formed may have as a core material which is solid, liquid, gas, or a multi-phasic compound. This invention is concerned with capsules having sizes ranging from approximately a micron to a few millimeters. Such capsules are generally referred to as microcapsules; although that term is not specifically defined in the literature. As used herein, the terms "shell" and "wall" are used interchangeably to denote the barrier surrounding the core material separating it from the environment.
Capsules and/or microcapsules need not be uniformly spherical but may consist of irregularly-shaped objects such as those having a shell surrounding an irregular shaped solid crystalline core. A capsule core may be a single solid crystal, a chemical compound, an emulsion, a liquid, a mixture of different solid materials or other suspensions, or it may be a combination of smaller capsules. The shell or wall may likewise be complex having multiple walls of different composition. Thus, it is possible to have a first capsule having its own core and shell which forms the core for a second capsule having a shell formed from the same or a different material.
Capsules have been developed to serve a variety of functions. One general purpose of encapsulation is to preserve or isolate the core material from its environment until an appropriate time or condition is present. In these situations, the core material is protected from the environment by the shell. Such protection is not always easily achieved since the core material may be able to penetrate or diffuse through the shell. On the other hand, use can be made of the "leaky" feature of some shells to control the release rate of the core material into the surrounding environment.
Encapsulation can also be used to protect compounds from environmental conditions such as temperature, pH, or chemically reactive surroundings such as oxidizing and reducing environments. Such oxidizing and reducing environments may consist of chemicals to which the capsule has been added. In other cases, it is desirable to encapsulate certain chemical compounds not only for protection of the core but also to protect or shield the external environment from reaction with the chemical compound forming the core. One common example of this use for encapsulation is the masking of the taste and/or odor of a chemical composition. In such a case, encapsulation may offer protection against detection of a bitter, toxic or otherwise undesirable taste or odor. Encapsulation of skin and respiratory irritants and toxins is one important way to protect the handlers of such materials from exposure.
During the past two decades, encapsulation of a broad range of materials has been achieved using a variety of encapsulation techniques. Capsules have found use in many applications such as in the manufacture of pharmaceuticals, pesticides, paints, adhesives and many other chemical products. To date, the most widely-known use of microcapsules has been in the product generally known as "carbonless paper". In carbonless paper, microcapsules provide a controlled release of a color forming reagent core. The forming agent is released from carbonless paper microcapsules when applied pressure ruptures the capsule walls. The agent then reacts with another dye forming member on the paper beneath to create an image.
Examples of processes for forming microcapsules are given in Vandegaer, "Microencapsulation Processes and Applications," Plenum Press, New York, 1974, M. Gutcho, "Microcapsules and other Capsules", Chemical Technology Review, No. 135, Noyles Data Service, Park Ridge, N.J. 1979, and the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition (1981), volume 15. Other references disclosing processes for forming microcapsules include U.S. Pat. Nos. 3,943,063; 3,460,972, 4,001,140; and 4,087,376.
The above-mentioned references describe several liquid-phase methods of encapsulation. These methods include coacervation, thermal coacervation, complex coacervation, interfacial polymerization, and others. In the process of coacervation, the core and shell materials are mixed together in a liquid medium. When the core and shell materials have been agitated for a sufficient period of time, portions of the core material become coated with shell material, thus forming capsules within the liquid medium. The size of these capsules is controlled by the speed and design of the mixing element within the vessel. The thickness of the shell material is adjusted by a further chemical treatment process.
FIG. 1 shows the process of coacervation, which is a liquid-phase microencapsulation process of the prior art. The details of this method are described in U.S. Pat. No. 2,800,457. In the method shown in FIG. 1, an oily substance, which comprises the core of the microcapsule, is dispersed in an aqueous solution of gelable hydrophilic colloid materials. The hydrophilic colloid materials, which become the shell of the capsules, are made to coagulate when the core material and the colloid materials are agitated within the aqueous carrier. Eventually, the emulsified droplets of the oily substance become coated with the colloid material, as the latter forms a solid wall or shell around each droplet. The capsules formed in this manner may be used in the liquid medium, or may be dried to a fine powder form.
Variations in the coacervation process have been developed. For example, polymers have been used as shell materials. It is possible to adjust the pH of the mixture to cross-link and harden the shell. However, both the method described above and its variations have disadvantages. A principal disadvantage of the prior art processes is the amount of time required to form capsules. The time consumed by a typical coacervation process is illustrated in FIG. 2. FIG. 2 shows the time required to complete the three major stages in capsule formation. As shown in FIG. 2, it takes about one hour to form "pre-capsules," i.e. newly-formed capsules which have very thin shells, and which need further hardening before they can survive in the outside environment. Microdispersions are examples of such materials. At this stage, the capsule walls occupy less than 5% of the volume of the capsules.
An additional two hours or more may be required to reach the second stage, wherein the shell is completely formed. At this point, additional layers of shell material are deposited onto the initial shell. In this second stage, the wall volume may be increased from 5% to above 90% of the total volume of the capsule, depending upon the duration of agitation, the level of turbulence of the agitation, and the concentration of shell material in the mixture.
The third stage of capsule formation, in the coacervation process, may require yet another 1-2 hours. In this stage, the shell is hardened into its final form. The hardening is often accomplished by cross-linking the shell material. The cross-linking is often induced chemically, or by adjusting the temperature of the completed capsules. Thus, as shown in FIG. 2, the time required for the entire coacervation process is several hours.
Listed below in Table 1 are the major encapsulation techniques of the prior art, showing the range of capsule sizes attainable with each technique, and indicating the phases of core materials which can be encapsulated with each technique. Coacervation has been described earlier. The other methods listed are described in Volume 15 of the Encyclopedia of Chemical Technology (1981), cited above, at pages 472-484.
TABLE 1 ______________________________________ MICROENCAPSULATION: PROCESS LIMITS CORE PROCESS MATERIAL SIZE (.mu.) ______________________________________ COACERVATION SOLID/LIQUID 10-500 INTERFACIAL ADDITION SOLID/LIQUID 5-2000 AND CONDENSATION AIR SUSPENSION SOLID 50-5000 CENTRIFUGAL EXTRUSION SOLID/LIQUID 250-3000 SPRAY DRYING SOLID/LIQUID 5-500 PAN COATING SOLID 500-5000 ______________________________________
Table 2 below lists some of the materials which can be encapsulated. However, this list is indicative only and is not meant to be inclusive.
TABLE 2 ______________________________________ MATERIALS WHICH CAN BE ENCAPSULATED ______________________________________ Activated carbons Enzymes Pesticides Adhesives Flame retardants Pharmaceuticals Amines Flavors Pigments Amino acids Food ingredients Reflective products Animal feed ingredients Fumigants Resins Antibiotics Inorganic salts Resin-curing agents Antiseptics Ion-exchange resins Retinoids Aqueous solutions Liquid hydro- Sealants carbons Catalysts Oils (vegetable) Sterilants Chemoluminescents Organometallic Steroids compounds Chlorinated hydrocarbons Oxidizers Vitamins Corrosion inhibitors Perfumes Water Deodorants Peroxides ______________________________________
The coacervation process described above has many disadvantages. It is difficult to achieve precise control of the size of the microcapsules. Inadequate agitation of the mixture frequently produces capsules which are too large, often beyond the size range suitable for the desired application. It is also difficult to adjust the thickness of the shell of the capsules. A thicker shell is often essential to enhance the shear and impact resistance of the capsule, and to enable the capsule to withstand high temperatures. In addition to these disadvantages, the coacervation process is also very time-consuming. The core and shell materials must be stirred for a long period of time, on the order of several hours, before usable capsules are produced. The time required to form the capsules adds significantly to the cost of their manufacture.
Conventional liquid-phase methods of making capsules, such as the coacervation process, often produce unsatisfactory quantities of encapsulated products. Moreover, it often happens that the core material is soluble in the liquid medium in which the shell is formed, in which case such materials dissolve in the liquid medium long before encapsulation can occur. There is presently a great demand for capsules which can be inexpensively manufactured, and which are suitable for various industrial applications.
Capsules used in industry must exhibit the following properties:
1. The capsules must be capable of withstanding large shear forces, or other stressful conditions, when the capsules are added to a host material. Suitable host materials could be paints, plastics, foam products, building materials, paper products and others. Each host material requires varying conditions of heat and stress to produce the final product, and the capsules must have suitable physical properties to enable the capsules to be used during the manufacture of the final product. PA0 2. Capsules used in industry must generally be very small. Microcapsules made by conventional liquid-phase methods of encapsulation, and by other methods, usually have an unacceptably wide size distribution, and are often too large for use in industrial processing. PA0 3. Capsules used in industry should be produced in a continuous process, so that the capsules are available in large quantities, and at relatively low cost.
The present invention provides a process and apparatus for making capsules which have the properties described above. The process of the present invention can produce capsules in a small fraction of the time required by conventional methods. The present invention also permits the accurate adjustment of the size of the capsules and the thickness of their shells.